A plastic optical part has merits of design facility and low manufacture cost, compared with a glass optical part with identical structure, so the plastic optical part has been recently utilized as an optical fiber, an optical lens element, an optical waveguide and so forth. Especially, a plastic optical fiber (referred to as “POF”), entirely composed of a plastic is suitable for manufacture of the optical fiber with large diameter at a low cost, because the POF has advantages in excellent flexibility, light weight and high machinability, compared with the glass optical fiber. Accordingly, it is planned to utilize the plastic optical fiber as an optical transmission medium for short-distance purpose in which the transmission loss is small.
The POF is composed of a core part, having organic compounds as the main component, in which a polymer forms a matrix, and a cladding part composed of organic materials having different refractivity from the core part. A graded index (GI) type POF, in which the refractive index in the core part gradually decreases from the center to the surface of the core part, has high transmission band and high transmission capacity. Various methods for manufacture of the GI type POF are disclosed (for example, Japanese Laid-Open Patent Publication (JP-A) No. 2002-220261 and U.S. Pat. No. 5,541,247 (counterpart of Japan Patent No. 3332922)). For example, the GI type POF is manufactured by forming an optical fiber base body (hereinafter referred to “preform”) by use of surface gel polymerization, and then by melt-drawing the preform.
When the preform contains microscopic bubbles during the manufacture process, such bubble becomes a cavity as an internal defect that extends in the lengthwise direction of the optical fiber strand after the melt-drawing process. Such cavities would cause decrease in transmission property and physical strength of the optical fiber.
In manufacture of the glass optical fiber, JP-A 2000-281379 discloses a defect detection apparatus provided with the extension machine so that the bubbles (internal detect) in the optical fiber is detected during the manufacture. This defect detection apparatus has a light emission member and a light detection member to detect the bubbles in the optical fiber. When the detection light from the light emission member is scattered by the bubble in the optical fiber, the light amount detected by the light detection member is decreased. Thereby, it is possible to detect the bubbles in the optical fiber. Also, in JP-A 2001-235396, laser beam from the light emitting member is scattered by the bubbles in the optical fiber, and the intensity of the scattered light is detected by an image sensor as the light detecting member. Based on the intensity distribution of the forward scatter, it is possible to detect the bubbles in the optical fiber.
In the method to detect a defect by use of forward scatter, as described in JP-A 2000-281379 and JP-A 2001-235396, the detection accuracy becomes worse since the diameter of the fiber strand of the POF is relatively large, compared with the glass optical fiber. For example, because of decrease in detection accuracy, the conventional defect detection apparatus can not detect microscopic bubbles having the diameter of 10-30 μm. Moreover, the method to detect defect by use of forward scatter will generate undetectable region in the fiber strand having large diameter.
An object of the present invention is to provide an apparatus and a method for detecting an internal defect in a plastic optical fiber with high precision.
Another object of the present invention is to provide a plastic optical fiber manufacturing apparatus that can detect the defect in manufacture.